A United States Patent US-2015/117865A has disclosed an optical received implemented within a coherent optical communication system. FIG. 20 schematically illustrates an optical coupling system disclose in the prior patent document above described. The optical receiver 100 in FIG. 20 provides an optical substrate 101, two optical hybrids, 111 and 112, photo-detectors, 134 and 135, and a monitor photodiode (mPD) 104, where those devices are enclosed within a housing 105. Signal light L1 and local light L2 enter respective optical waveguides, 106 and 107, through an end 101a of the optical substrate 101.
A beam splitter (BS) 131 splits the signal light L1 into two parts, one of which enters the PD 104, while, another advances another BS 132 that splits the signal light L1 coming from the BS 131 also into two parts that enter respective optical hybrids, 111 and 112. On the other hand, the local light L2 is also split by still another optical splitter 133 to the respective optical hybrids, 111 and 112. Thus, the optical coupling system that optically couples the signal light L1 and the local light L2 with the optical hybrids, 111 and 112, provides the optical components, 131 to 133, with optical surfaces whose normals make angles against an optical axis of the signal light L1 and/or the local light L2 except for 0° and 90°. In order to enhance the coupling efficiency of the light, L1 and L2, with the optical hybrids, 111 and 112, the surfaces of the optical components, 131 and 132, exactly, the normals thereof are necessary to be precisely aligned with the optical axes of the light, L1 and L2, as keeping a preset angle different from 0° and 90°.